Translate organic structural and displayed formulas
Read condensed structural and displayed formulas by preserving atom connectivity and functional-group attachment.
Section 29 · An introduction to A Level organic chemistry
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 29. Follow the formula, naming and optical-isomerism limits stated in the syllabus. Benzene-ring displayed formulas are not expected; use the supplied convention for aromatic skeletal formulas. Do not extend the optical-isomerism scope to naming diastereoisomers or meso compounds. Written practice is self-checked
Before you begin
- Use covalent formula conventions, functional groups, reaction terminology, and the AS organic chemistry foundations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a displayed formula make explicit?
What you will learn
- Distinguish general, structural and displayed formulas
- Read atom connectivity from condensed groups
- Check valency and functional-group attachment when translating formulas
Each formula representation carries different detail
A general formula describes a pattern for a family; a structural formula shows how atoms or groups are connected in a condensed form; a displayed formula shows individual atoms and bonds. They can represent the same molecule at different levels of detail.
Translate by preserving connections. In CH₃CH₂COCl, the carbonyl carbon is bonded to CH₂ and Cl. Writing CH₃COCH₂Cl instead changes the connectivity and produces a different class of compound.
Use valency as a check
Carbon normally forms four covalent bonds, oxygen two and hydrogen one in these neutral structures. Nitrogen bonding depends on its class; check the number of carbon groups and hydrogens rather than assuming all nitrogen compounds are alike.
For aromatic molecules, Cambridge does not expect benzene-ring displayed formulas to be drawn. Use the convention given for the ring and show the attached substituents clearly.
Pause and explain
In CH₃CH₂COCl, which group is bonded to the carbonyl carbon besides chlorine?
Worked example
Interpret CH₃CH₂COCl and state which atoms are connected to the carbonyl carbon.
Show the worked solution
- Identify the COCl ending as C(=O)Cl.
- The carbonyl carbon is preceded by CH₂ and bonded to Cl.
- The structure is CH₃–CH₂–C(=O)–Cl.
Answer The carbonyl carbon is bonded to the ethyl group and to chlorine; the molecule is propanoyl chloride.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write a clear condensed structural formula for butanoyl chloride and identify its functional group.
Give me a hint
Use a four-carbon chain ending in –C(=O)Cl.
Compare my reasoning
- Count four carbons including the carbonyl carbon.
- Place the carbonyl at the end of the chain.
- Attach Cl to the carbonyl carbon.
CH₃CH₂CH₂COCl; it is an acyl chloride.
Look for these in your work
- Four carbons counted
- Connectivity preserved
- Functional group named
A student writes CH₃CH₂COCH₃ for butanoyl chloride. What is wrong with the formula?
Give me a hint
Compare the formula's carbonyl neighbours with the required –COCl group.
Compare my reasoning
- The written formula has carbon groups on both sides of C=O.
- It contains no chlorine.
- It is a ketone pattern, not an acyl chloride.
It is a ketone-like structure with no chlorine; butanoyl chloride should be CH₃CH₂CH₂COCl.
Look for these in your work
- Connectivity checked
- Missing Cl identified
- Correct pattern supplied
Translate the condensed part –CONHCH₃ into a connectivity description and classify it.
Give me a hint
Expand the carbonyl and identify the group bonded directly to its carbonyl carbon.
Compare my reasoning
- Write –C(=O)–NH–CH₃.
- Nitrogen is bonded to the carbonyl carbon and carries one H and one methyl group.
- That is a secondary amide.
The sequence is –C(=O)–NH–CH₃; it is a secondary amide because N is bonded to the acyl carbon, one methyl group and one H.
Look for these in your work
- Carbonyl expanded
- N substituents identified
- Amide class correctly assigned
Common mistakes
- Reordering groups and changing which atoms are bonded
- Treating a condensed formula as a molecular formula
- Adding a benzene displayed formula when the syllabus does not expect it
- Forgetting carbon valency when checking a translated formula
What can you explain now?
What does a displayed formula make explicit?
Compare with the explanation
The atoms and the bonds between them
A displayed formula shows individual atoms and their connections.
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Make it stick
How can you check that translating a condensed formula has preserved the molecule rather than changed its connectivity?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.